1. Field of the Invention
The present invention relates to a method of forming film, in which dopes such as P, B, As and/or Sb are included, on substrates.
In order to make characteristics of IC chips stable (or to cause these IC chips to have desired electric characteristics), it is usually needed that desirable uniform film is formed on semiconductor wafers in the film forming process. It is therefore important in the film forming process that irregularity in film thickness in the surface of each semiconductor wafer is reduced to an extent as greater as possible (which will be hereinafter expressed by that "the uniformity of thickness of the film formed on each wafer is increased"). It is also important that irregularity in film thickness in surfaces of a plurality of batch-processed semiconductor wafers is reduced to an extent as greater as possible (which will be hereinafter expressed by that "the uniformity of the same thickness of films formed on the wafers is increased").
Particularly in a case where doped film is to be formed on a plurality of semiconductor wafers according to the batch processing manner, it is important that irregularity in dope concentrations in films formed on the wafers is reduced to an extent as greater as possible (which will be hereinafter expressed by that "the uniformity of the same dope concentrations in films formed on the wafers is increased").
Further, in order to make the cost lower, it is needed that the throughput at each manufacturing process is enhanced. The film developing speed (or film forming speed), for example, is increased to enhance the throughput at the film forming process.
In a case where mixed gas of diluting phosphine (PH.sub.3) and pure silane (SiH.sub.4) is used to form P-doped polysilicon film on the wafers, the mixed gas is introduced into the process tube at a flow amount of 100-200 SCCM and reacted with the silicon wafers at a temperature of 600.degree.-630.degree. C. A film forming speed of about 25 .ANG./min. can be thus achieved. The film forming reaction can be expressed as follows. EQU SiH.sub.4 .rarw..fwdarw.SiH.sub.2 +H.sub.2 EQU SiH.sub.2 .fwdarw.Si +H.sub.2
The P-doping gas phosphine is highly reactive. Phosphor is therefore liable to primarily adhere to the surface of each silicon wafer before silane does. The possibility that silane and adhere to the surface of each wafer is thus made low to thereby reduce the throughput. The film forming speed is lowered by 5 .ANG./min. Further, the uniformity of thickness of the film formed on each wafer is increased by .+-.25% to thereby lower the productivity of wafers.
In order to increase the uniformity of the same thickness of the film formed on each wafer, it is imagined that the interval between the two adjacent wafers in the process tube is made large enough, but when doing so, the number of wafers which can be batch-processed one time becomes smaller to thereby lower the throughput, too.
Japanese Patent Disclosures Sho 58-108735 and Sho 61-201695 disclose wafer boats of the vertical type in which the film forming process can be carried out so as to provide a high uniformity in the thickness of the film formed on each wafer but without making the interval between the adjacent wafers too large.
In the case of these vertical type wafer boats, however, the ring member for supporting the wafers becomes complicated in shape, thereby making the cost high. In addition, the maintaining of the wafer boats becomes complicated, too. Further, the film forming speed is still left slow. Furthermore, the uniformity of the same thicknesses of films formed on the wafers cannot be increased. The uniformity of the same dope concentrations in films formed on the wafers can be increased neither.
Usually, the film forming temperature may be raised in order to make the film forming speed high. However, the amount of P doped into the thin film is decreased under high process temperature.
Further, when the film forming temperature is lowered while keeping the amount of silane supplied as described above, the film forming speed becomes far lower than the speed practically used.